A typical intercom has an amplifier, two or more stations each with a microphone and speaker or a two-way (microphone or speaker) transducer and a switch interconnecting the microphone, speaker and transducer with the amplifier for communication between the two stations. This invention is concerned particularly with an intercom where two stations are so related physically that audio feedback from the speaker at one station is coupled to the microphone of the other station, causing oscillation.
The oscillation problem has been found to be particularly troublesome with a two station intercom for a kiosk, as at a self-service gasoline facility, or a drive-up window at a bank or fast food establishment. An intercom station for the attendant in the kiosk has a microphone and speaker. A second station, outside the kiosk, has a transducer that serves both as microphone and speaker. The transducer is typically mounted in or on the wall of the kiosk. A switch, for example a voice operated switch controlled by the attendant, connects the first station microphone with the input of the amplifier and the second station transducer with the amplifier output when the attendant in the kiosk speaks, for communication from the attendant at the first station to the customer at the second station. Alternatively, when the attendant is not speaking at the first station, the switch connects the second station transducer with the amplifier input and the first station speaker to the output of the amplifier for communication from the customer to the attendant. In both conditions the intercom is subject to oscillation unless the first station has a proximity microphone, in which case oscillation is more likely to occur only when the customer is speaking to the attendant. Many installations do not, however, use a proximity microphone as they are more expensive than other microphones, and the user must be close to the microphone when speaking. Other intercom configurations are also subject to oscillation and some examples will be described below.
The air feedback system in which the intercom operates has physical elements, including the kiosk or other structure air spaces or columns and the mechanical mounting for the transducer, which are subject to mechanical resonance at one or more frequencies in the audio spectrum. These mechanical resonances reinforce the feedback energy and contribute to intercom oscillation. It is impractical to damp the mechanical vibrations of all of the physical elements which affect the intercom.
An attempt to reduce intercom oscillation by using a moving cone transducer (microphone/speaker) with a solid metal frame, isolating the rear surface of the cone, had modest success. However, this transducer construction caused a severe deterioration of audio quality.